Japanese patent application publication No. 2010-162318 (hereinafter referred to JP '318) discloses a sole structure for a shoe with a leaf spring structure that is composed of a hard treading plate extending from the heel portion to the forefoot portion and a hard ground-contact plate located away from the treading plate and extending from the heel portion to the forefoot portion and in which the treading plate and the ground-contact plate are coupled to each other at the heel portion (see para. [0010] and FIG. 1). According to the description of JP '318, when an external force to narrow a gap between the treading plate and the ground-contact plate is applied, the leaf spring structure generates a restoration force thereby causing a strong kick at the time of push-off motion of the foot (see para. [0018] and FIGS. 2-5).
WO2006/129837 (hereinafter referred to WO '837) discloses a sole structure that comprises an upper plate disposed on an upper side and extending from the heel region to the forefoot region, a lower plate disposed under and coupled to the upper plate via an elastic block and having a wavy corrugated shape formed of two downwardly convexly protruding parts at the heel region and an undulation at the forefoot region (see line 19 of page 10 to line 16 of page 14, and FIGS. 1A, 1B and 2). According to such a sole structure, at the time of striking onto the ground, a void formed at the heel region can generate cushioning properties and the wavy corrugated shape of the lower plate can improve bending properties.
In the sole structure described in JP '318, during the push-off motion of the foot, a gap between the treading plate and the ground-contact plate at the toe portion is narrowed, and the heel portion of the shoe is disengaged from the treading plate (see para. [0018] and FIG. 4). At this juncture, as can be seen from comparison between the shoe in FIG. 4 and the shoes in FIGS. 1-3 and 5, in the state immediately before the push-off motion of the foot shown in FIG. 4, a forefoot region of an shoe upper bends. Because unless the forefoot region of the shoe upper bends the gap (see FIG. 4) is not created between a heel region of the shoe upper and the treading plate. Therefore, in the structure of JP '318, during the push-off motion of the foot, as the forefoot region of the shoe upper bends MP joints (i.e. Metatarsophalangeal joints) of the foot bends.
In the sole structure described in WO '837, since the lower plate has the undulation formed at the forefoot region, during the push-off motion of the foot, as the lower plate bends along the undulation the forefoot region bends. Therefore, in the structure of WO '837 as well, during the push-off motion of the foot, as the forefoot region bends the MP joints of the foot bends.
At this point, especially, in the case of a person of a large constitution, when such a weighty person runs, if he/she uses MP joints during every push-off motion of the foot, the burden on the MP joints becomes large. Also, when bending the MP joints he/she uses a knee joint as well, thus increasing the burden on the knee joint too. On the other hand, it is considered that when running slowly, if there is not a problem of the interconnection with the motion of the foot, the burden on the MP joints and the knee joint can be reduced by conversely restraining bending of the MP joints. In this case, if cushioning properties of the forefoot region are insufficient there is a risk that foot joints and the knee joint are injured.
The present invention has been made in view of these circumstances and its object is to provide a sole structure for a shoe that can reduce the burden of MP joints and a knee joint of a shoe wearer's foot during running and that can improve cushioning properties. Also, the present invention is directed to providing a sole structure for a shoe in which especially a person of a large constitution can run without imposing a burden on the body when he/she runs slowly.
Other objects and advantages of the present invention will be obvious and appear hereinafter.